CDROM

CDROM an abbreviation of Compact Disc readonly memory is a Compact Disc that contains data accessible by a computer. While the Compact Disc format was originally designed for music storage and playback the format was later adapted to hold any form of binary data. CDROMs are popularly used to distribute computer software including games and multimedia applications though any data can be stored up to the capacity limit of a disc. Some CDs hold both computer data and audio with the latter capable of being played on a CD player whilst data such as software or digital video is only usable on a computer such as PC CDROMs. These are called Enhanced CDs. Although many people use lowercase letters in this acronym proper presentation is in all capital letters with a hyphen between CD and ROM. It was also suggested by some especially soon after the technology was first released that CDROM was an acronym for Compact Disc readonlymedia or that it was a more correct definition. This was not the intention of the original team who developed the CDROM and common acceptance of the memory definition is now almost universal. This is probably in no small part due to the widespread use of other ROM acronyms such as FlashROMs and EEPROMs where memory is the correct term.

          Compact Disc

CDROM discs are identical in appearance to audio CDs and data is stored and retrieved in a very similar manner only differing from audio CDs in the standards used to store the data. Discs are made from a. mm thick disc of polycarbonate plastic with a thin layer of aluminum to make a reflective surface. The most common size of CDROM disc ismm in diameter though the smaller Mini CD standard with anmm diameter as well as numerous nonstandard sizes and shapes. business cardsized media are also available. Data is stored on the disc as a series of microscopic indentations pits with the gaps between them referred to as lands. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands. Because the depth of the pits is approximately onequarter to onesixth of the wavelength of the laser light used to read the disc the reflected beams phase is shifted in relation to the incoming beam causing destructive interference and reducing the reflected beams intensity. This pattern of changing intensity of the reflected beam is converted into binary data.

          Standards

There are several formats used for data stored on compact discs known collectively as the Rainbow Books. These include the original Red Book standards for CD audio White Book and Yellow Book CDROM. The ECMA standard which gives a thorough description of the physics and physical layer of the CDROM inclusive of CIRC and EighttoFourteen Modulation can be downloaded from. ISOdefines the standard file system of a CDROM although it is due to be replaced by ISO. UDF format is used on userwriteable CDR and CDRW discs that are intended to be extended or overwritten. The bootable CD specification to make a CD emulate a hard disk or floppy is called El Torito. Apparently named this because its design originated in an El Torito restaurant in Irvine California.

          CDROM format

A CDROM sector containsbytes divided intobyte frames. The CDROM is in essence a data disk which cannot rely on error concealment and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection a CDROM has a third layer of ReedSolomon error correction. A Mode CDROM which has the full three layers of error correction data contains a netbytes of the availableper sector. In a Mode CDROM which is mostly used for video files there areuseravailable bytes per sector. The net byte rate of a Mode CDROM based on comparison to CDDA audio standards is. ks×B× . kBs. The playing time isminutes orseconds so that the net capacity of a Mode CDROM is MB.

          CD sector contents

A standardmin CD containsblocks or sectors. Each sector isbytes and containsbytes of PC MODE Databytes of PSXVCD MODE Data orbytes of AUDIO. The difference between sector size and data content are the Headers info and the Error Correction Codes that are big for Data high precision required small for VCD standard for video and none for audio. If extracting the disc in RAW format standard for creating images always extractbytes per sector notbytes depending on data type basically extracting the whole sector. This fact has two main consequences Recording data CDs at very high speed x can be done without losing information. However as audio CDs do not contain a third layer of error correction codes recording these at high speed may result in more unrecoverable errors or clicks in the audio. On aminute CD one can fit larger images using RAW mode up to× bytesMiB. This is the upper limit for RAW images created on amin orMiB Red Book CD. The. increase is due to the discarding of error correction data Please note that an image size is always a multiple ofbytes the size of a block when extracting in RAW mode.